Waste management has always been a big problem in big cities. Fortunately through several environmental initiatives, communities have begun to separate dry and wet waste matter. The dry waste primarily comprises of natural dead leaves and similar waste as well as plastics. Dead leaves type of waste (including rice husk and other dry agricultural side products) is in fact available in plenty in woods as well as agricultural farms in villages. Most of this type of waste is a rich source of carbon but may contain other elements in different proportions depending on the source of the waste. Usually the waste from natural sources is just burnt producing ash and hazardous gaseous pollution products. There have been some initiatives to employ the ash but in most situations the use is in the form of passive fillers. If the natural waste and some of the man-made waste is harnessed to synthesize functional forms of carbon, one would get a high value-added product for diverse carbon based applications that have been growing rapidly in the past decade including super-capacitors, batteries, super-adsorbents for gases and toxins etc.
Carbon is the most naturally occurring abundant material present on earth exhibiting variety of molecular and structural forms such as graphite, diamond, nanotubes, graphene, fullerene, nano-diamonds, amorphous carbon, porous carbon etc. which have tremendous applications in various fields. Due to their high surface area, high mechanical strength, electrical, thermal and optical properties these forms have applications in super capacitor, battery, catalysis and other fields. Carbon has also been used in the form of nanocomposites with metals, metal oxides, nitrides, carbides, semiconductors etc. Researchers have tried several synthetic techniques to get high quality carbon such as carbonization of organic/polymeric precursors, autoclave synthesis from small halogenated aromatic molecules, chemical vapour deposition, excimer laser ablation of graphitic targets, sputtering/plasma methods, arc discharge methods, chemical methods (autoclave) etc. Apart from these techniques scientists have now started utilizing organic waste material such as food, agricultural wastes, and insects for the synthesis of carbon in grapheme.
The use of plant leaves for removal of water soluble dyes such as methylene blue, brilliant green, congo red and methylene blue is reported in the literature, such as K. G. Bhattacharyya in Indian journal of chem Tech. 12, 2005, 285-295 discloses utilization of a biosorbent based on Azadirachta indica (Neem) leaves for removal of water-soluble dyes. Article titled “Removal of Direct Red Dye Using Neem Leaf Litter” by Sivamani S et al. in Helix Vol. 1(2):129-133 (2012) reported the adsorption of Congo red (CR) onto carbon prepared from Neem leaf litter.
Further Neha Gupta in Journal of the Taiwan Institute of Chemical Engineers 43, (4), July 2012, 604-613 discloses, batch adsorption using ashoka (Saraca asoca) leaf powder (ALP), as an adsorbent for the removal of cationic dyes such as methylene blue, malachite green, rhodamine B and brilliant green from aqueous solution. Dharmendra Singh et al in Inventi Rapid: Water & Environment Vol. 2011, 3 discloses removal of color from aqueous solution by using low cost adsorbent Azadirechta indica Leaves”.
The pyrolysis of neem (Azadirachta indica) and kikar (Acacia arabica) leaves at 400° C. in electric furnace for the preparation of carbon for fluoride removal is reported in J. Environ Biol. 2008 March; 29(2):227-32 by Kumar S, et al.
Moreover Bhardwaj S. in Carbon Letters. 8 (4) (2007) 285-291 discloses the synthesis of carbon materials by pyrolysis of Soap-nut seeds (Sapindus mukorossi), Jack Fruit seeds (Artocarpus heterophyllus), Date-seeds (Phoenix dactylifera), Neem seeds (Azadirachta Indica), Tea leaves (Ehretia microphylla), Bamboo stem (Bambusa bambus) and Coconut fiber (Cocos nucifera), without using any catalyst. Amongst the various precursors, carbon fibers obtained from Soap-nut seeds (Sapindus mukorossi) and Bamboo stem (Bambusa bambus), even after 100th cycles, showed the highest capacity of 130.29 mAh/g and 92.74 mAh/g respectively.
Further Bhardwaj et al. (Asian J. Exp. Sci. 22 2008; 89-93) have synthesised carbon nanomaterial from tea leaves as an anode in Lithium secondary batteries. However the leaves used in the citation are tea leaves which have high commercial value. The current invention is based on the synthesis of highly conducting carbon and its nanocomposites with metal and metal oxides from readily available dead leaves. The process of the citation involves boiling leaves for about 1 hr. then dried at 100° C. for 3 hrs, washing several times and finally pyrolysis.
While in the current invention the leaves are washed, dried directly and then pyrolysed. Hence the invention is advantageous as compared to methods of prior art as it uses dead leaves instead of any starting material of commercial value and provide shorter process for synthesis of carbon nanomaterials employing lesser energy.
The functional carbon is normally synthesized by different chemical methods following complex steps using man-made chemical precursors, such as synthesis of functional carbon (graphene) by chemical vapor deposition using different waste materials such as lignocellulosic biomass, food, insects as the source of precursor vapor phase materials. In present case functional carbon forms as the residue of the pyrolysis process when the vapor phase precursors are removed, making instant process distinctly different as compared to what is reported. Further instant invention makes it possible to synthesize various nanocomposites in bulk form by our process, which cannot be the case with the reported publication. Moreover there are no reports on the use of plant dead leaves as a source of conducting carbon.
In view of above the present inventor addresses the technical constraints associated with the existing process such as cumbersomeness, toxicity of reagents, limited application, energy and time consuming, environmentally hazardous by-products, low surface area and capacitance of derived carbon material, expensive starting material etc. To overcome such problems the inventors have developed source of carbon nanomaterial by industrially feasible, cost-effective technique prepared from cheap, non-toxic, bio waste material such as plant dead leaves, that affords carbon nanomaterial with significant properties. Further the present inventors have succeeded to solve the problem related to waste material and pollution by employing waste organic material for the synthesis of valuable functional and device-worthy carbon materials.